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SPI significat servitium provisori interface, quod intelligi potest interfacem specifice provisoribus vel ecomidibus qui ad functiones compages extendunt. SPI servitium interfaciendi separat ab exsecutione servitii specifici, ministerium salutatoris et instrumenti operantis decoquitur, et scalabilitatem et conservabilitatem progressionis emendare potest. Modificare vel reponens ministerium exsecutionis non requirit modificando RECENS. Plures compages utuntur Java's SPI mechanismo, ut: Artificium vernum, aurigae oneraturae datorum, interfacei stipes, et exsecutionem extensio Dubbo, etc. Refer ad mechanismum Dubbo SPI ad efficiendum SPI partem compagis huius RPC.

Exempli causa, cliens cum servo communicat, nuntium serialize necesse est. Multae sunt algorithm serializationis quae in serializatione adhibentur, inter Hessian, Kryo et ProtoStuff. Ratio postulandi est methodum vocare in classe ad algorithmum serialization pertinens respondens secundum nomen algorithmus serialization in nuntio ad faciendam serializationem et deserializationem .

SPI ita utitur:

Serializer serializer = ExtensionLoader.getExtensionLoader(Serializer.class)
                    .getExtension(codecName);

codecName nomen est algorithmus serializationis et correspondens classis secundum hoc nomen onerari debet.

    private final Class<?> type;
 
    private ExtensionLoader(Class<?> type) {
        this.type = type;
    }
 
    // 每个SPI接口都有自身的ExtensionLoader
    public static <S> ExtensionLoader<S> getExtensionLoader(Class<S> type) {
        if (type == null) {
            throw new IllegalArgumentException("Extension type should not be null.");
        }
        if (!type.isInterface()) {
            throw new IllegalArgumentException("Extension type must be an interface.");
        }
        if (type.getAnnotation(SPI.class) == null) {
            throw new IllegalArgumentException("Extension type must be annotated by @SPI");
        }
        // firstly get from cache, if not hit, create one
        ExtensionLoader<S> extensionLoader = (ExtensionLoader<S>) EXTENSION_LOADERS.get(type);
        if (extensionLoader == null) {
            EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<S>(type));
            extensionLoader = (ExtensionLoader<S>) EXTENSION_LOADERS.get(type);
        }
        return extensionLoader;
    }

Quisque SPI interface suam ExtensionLoader habet. Cum vocans getExtensionLoader, primum seriem actionum reprehendo legalium perficiet, et deinde ExtensionLoader interfaciendi impetrare conetur adeptus, creare Loader object.

Deinde, instantia per getExtensionem obtinetur, et instantia etiam localiter conditiva est.

    private final Map<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<>();
 
    public T getExtension(String name) {
        if (StringUtil.isBlank(name)) {
            throw new IllegalArgumentException("Extension name should not be null or empty.");
        }
        // firstly get from cache, if not hit, create one
        // 缓存holder
        Holder<Object> holder = cachedInstances.get(name);
        if (holder == null) {
            cachedInstances.putIfAbsent(name, new Holder<>());
            holder = cachedInstances.get(name);
        }
        // create a singleton if no instance exists
        // holder为空，双重检查锁创建示例
        Object instance = holder.get();
        if (instance == null) {
            synchronized (holder) {
                instance = holder.get();
                if (instance == null) {
                    instance = createExtension(name);
                    holder.set(instance);
                }
            }
        }
        return (T) instance;
    }

Obiecto Classis obiecto classis, hoc per reflexionem potes creare.

   // 缓存   
   private static final Map<Class<?>, Object> EXTENSION_INSTANCES = new ConcurrentHashMap<>();
 
   private T createExtension(String name) {
        // load all extension classes of type T from file and get specific one by name
        // SPI接口对应的实现类，其标识名与class文件的映射，根据标识名获取class
        Class<?> clazz = getExtensionClasses().get(name);
        if (clazz == null) {
            throw new RuntimeException("No such extension of name " + name);
        }
        T instance = (T) EXTENSION_INSTANCES.get(clazz);
        if (instance == null) {
            try {
                // 缓存中不存在，则创建实例
                EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
                instance = (T) EXTENSION_INSTANCES.get(clazz);
            } catch (Exception e) {
                log.error(e.getMessage());
            }
        }
        return instance;
    }

Clavis est processus obiecti Classis obtinendi, id est, methodus getExtensionCalsses;

    // 该SPI接口所有实现类的标识与其Class对象的缓存
    private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<>(); 
 
    private static final String SERVICE_DIRECTORY = "META-INF/extensions/";  
 
    private Map<String, Class<?>> getExtensionClasses() {
        // get the loaded extension class from the cache
        // 根据Interface实现类的类名获取对应类的缓存
        Map<String, Class<?>> classes = cachedClasses.get();
        // double check
        if (classes == null) {
            synchronized (cachedClasses) {
                classes = cachedClasses.get();
                if (classes == null) {
                    classes = new HashMap<>();
                    // load all extensions from our extensions directory
                    loadDirectory(classes);
                    // 将Map集合存储在Holder中进行缓存
                    cachedClasses.set(classes);
                }
            }
        }
        return classes;
    }
 
    private void loadDirectory(Map<String, Class<?>> extensionClasses) {
        // 固定路径下的文件，SPI接口的类名作为文件名，在此文件中规定需要加载的实现类
        String fileName = ExtensionLoader.SERVICE_DIRECTORY + type.getName();
        try {
            Enumeration<URL> urls;
            // 系统类加载器，它能够加载用户类路径（ClassPath）上的类和资源。对于SPI机制尤为重要，因为SPI的实现类通常是由应用程序提供并放置在应用程序的类路径下的
            ClassLoader classLoader = ExtensionLoader.class.getClassLoader();
            // 获取当前类加载器加载的URL资源，文件名确定一般urls是唯一的
            urls = classLoader.getResources(fileName);
            if (urls != null) {
                while (urls.hasMoreElements()) {
                    URL resourceUrl = urls.nextElement();
                    // 使用classLoader加载资源，资源目标在resourceUrl下，加载后的class存储在extensionClasses Map集合当中
                    loadResource(extensionClasses, classLoader, resourceUrl);
                }
            }
        } catch (IOException e) {
            log.error(e.getMessage());
        }
    }
 
    private void loadResource(Map<String, Class<?>> extensionClasses, ClassLoader classLoader, URL resourceUrl) {
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(resourceUrl.openStream(), UTF_8))) {
            String line;
            // read every line
            // #是注释，截取注释之前的部分
            while ((line = reader.readLine()) != null) {
                // get index of comment
                final int ci = line.indexOf('#');
                if (ci >= 0) {
                    // string after # is comment so we ignore it
                    line = line.substring(0, ci);
                }
                line = line.trim();
                if (line.length() > 0) {
                    try {
                        final int ei = line.indexOf('=');
                        // 标识与类名
                        String name = line.substring(0, ei).trim();
                        String clazzName = line.substring(ei + 1).trim();
                        // our SPI use key-value pair so both of them must not be empty
                        if (name.length() > 0 && clazzName.length() > 0) {
                            // 加载类
                            Class<?> clazz = classLoader.loadClass(clazzName);
                            // 在map中保存
                            extensionClasses.put(name, clazz);
                        }
                    } catch (ClassNotFoundException e) {
                        log.error(e.getMessage());
                    }
                }
 
            }
        } catch (IOException e) {
            log.error(e.getMessage());
        }
    }

kyro=github.javaguide.serialize.kyro.KryoSerializer
protostuff=github.javaguide.serialize.protostuff.ProtostuffSerializer
hessian=github.javaguide.serialize.hessian.HessianSerializer

In methodo lavacrum vocationis cognoscit processum onerandi SPI conformationis tabellae sub META-INF/extensionibus/ iter ad onerandum objectum Classis, et instantiam obtine.

Animadvertendum est tabellam nomen sub META-INF/extensionibus/ necesse esse consentaneum esse cum codice. Contentum in tabella etiam scribendum est secundum (explicationem classis identifier = genus plenum genus nomine exsecutionis), ut consentaneum esse possit cum codice et programma recte parse tabellae et oneratus classi utatur. oneratis respondentem Ed. Demum, cache secundum <exsecutionem classis identifier, exsecutionem classis obiectum Classis>.

Ob exsistentiam trium cacherium (genus identifier, classis obiectum), (Class object, exempli objecti), et (genus identifier, instantiae objecti), directe transire potes in identifier ad obtinendum instantiam respondentis classis, quae etiam RPC compage optimizes.